1. Field of the Invention
This invention relates to a method used to form an article of manufacture comprising a container having an easily peelable clamped-wave container/lid seal system.
2. Discussion of the Prior Art
As discussed in U.S. Pat. No. 3,491,935 packages of thermoplastic materials have been used extensively in various industries for dispensing a wide variety of both flowable and non-flowable materials. These packages consist of containers that have been sealed in various ways, and recently there has been a great need for a container sealed with thin-wall sealing material that can be easily removed by the consumer to access the contents of the sealed package.
The '935 patent uses a differential method of forming a peelable seal for the containers in which the two thermoplastic materials selected to form the lid and the container have different softening points. These materials are heated while they are in contact with one another so that the first material fuses and secures itself to the second material, and thereby forms the desired peelable seal without fusing the second material (lines 15-22 of col. 2).
In the sealing operation of the '935 patent, the heat flows through the closure material from a heated sealing die which also extrudes a portion of the closure material from beneath the die, so as to form a bead on the flange of the container. It should be noted that the beads formed in this reference are only made of the lid polymer, are not on the inner rim of the flange, and also that a combination clamp head and seal head apparatus is not used to form the seal.
U.S. Pat. No. 4,519,499 issued to Stone et al on May 28, 1985, (as well as U.S. Pat. No. 4,496,046, same inventor and assignee), discloses a container having a selectively openable seal line and peelable barrier means. The container is made of two sheets of the same or similar material which are secured to each other by a seal that includes a bead area of increased thickness at the break end of it, (FIGS. 1, 2 and 3; lines 46-48 of col. 2). The sealing die used for sealing the two sheets together consist of a die head and a mirror image die head. Each of them has a convex projection and a bevel surface formed 5 degrees from horizontal (FIGS. 7, 8, 9, and 10, lines 15-32 of col. 6). This reference relates to the use of a sealing die having a beveled surface and the formation of beads having the same or similar melting temperatures. However, it does not relate to the formation of beads of different material on the inner rim of a container, or the use of a combination seal head and clamp head to form a clamped-wave seal structure.
U.S. Pat. No. 3,679,509 issued to Fielibert on Jul. 25, 1972 relates to a process for sealing laminated materials. This reference uses a V-shaped bar as its sealing head in which the obtuse angle ranges from 150.degree.-170.degree. (lines 14-21 of col. 2). The use of a clamp head in combination with a seal head is not disclosed.
U.S. Pat. No. 4,171,236 discloses a method for forming a fractionable seal of the type wherein a plastic cup-shaped closure is hermetically sealed to a plastic planar surface, such as the neck of pharmaceutical solution containers. The seal is formed with the closure by forcing a die heated above the melting point of the plastic materials into the desired planar surface. Both the planar surface and the closure adjacent thereto become molten and the surface material is displaced to form a fused seal transverse to the surface. Due to the design of the die, a frangible section is formed in the vertical sidewall of the depression. This reference does not disclose the formation of any beads on the inside rim of a container flange, nor the use of a clamp head to prevent molten lid/container material from flowing toward the outside rim of a container.
U.S. Pat. No. 4,961,513 issued Oct. 9, 1990 to Gossedge et al, (UK Patent Application GB 2,213,125A), discloses the formation of beads on both the inside and the outside rims of a container flange, but the failure plane formed during removable of the lid from the container is defined in a complex manner through both the lid and the container flange material. Gossedge's complex failure plane design, which is dependent on the "thinness" of ears projecting from the container flange material for successful operation, can understandably lead to admittedly "unacceptable", (FIG. 5 of Gossedge), failure plane variances such that " . . . the HDPE layer 12 will tear and thereafter delaminate from the foil 14 starting at the point B, in which case satisfactory peeling of the closure away from the rim will not have been achieved." (column 4 lines 17 through 48 of Gossedge). More importantly, the Gossedge reference does not teach the use of a clamp head in combination with a seal head, or the formation of a bead only on the inside rim of the container so as to reduce peel initiation forces.
In food packaging, the area of the invention, it is important that the lid remain fixed to the container during distribution of the closed container via various transportation routes to the eventual consumer. In an attempt to standardize the survivability determination of sealed containers, the U.S.D.A. has defined the "Restrained Burst Test" as a means of measuring this ability to maintain package integrity. The restrained burst test involves pressurizing the inside of the subject container with compressed air. The sealed container must be able to maintain +5 psig between the inside and outside of the container for at least 60 seconds.
Unfortunately, when one increases the survivability of the package by increasing the width and strength of the seal area between the container and the lid, (such as to withstand a 10 psig pressure differential), the peel strength in the prior art devices also increases in a linear manner such that it becomes very hard for the purchaser of the packaged food container to open the lid. The designer of the food container is therefore faced with a Hobson's choice, wherein a container that may survive the transportation phase will not be openable by the subsequent purchaser, or where an "easy open" container will not survive the transportation process. Increasing the burst strength of a container causes an undesirable increase in the peel strength required to open the container.
Most food packers require a minimum of 10 psi burst pressure and would like 15 psi. With conventional lid sealing, 15 psi bursts requires a material that has a peel strength of 10.75 lbs/in. This is extremely difficult to peel.
As disclosed in the above-referenced first two co-pending patent applications, a simple "interlocking wave" seal structure was developed that gave the sealed container a high burst pressure, yet allowed the lid to be peeled easily from the container, once the lid peeling process was started. In other words the average peel force necessary to continuously pull the lid from the container was reduced below that encountered in the conventional lid/container adhesive sealing systems. (Please reference FIGS. 5A, 5B, and 5C of this specification for both pictorial and graphic description of the simple wave seal structure).
The force required to start peeling the lid from the container, (the peel initiation force), however tended in some instances to be slightly higher than that encountered during the peel initiation of a conventional lid/container adhesive seal system, (reference FIG. 5C), even though the average peel force was substantially lower.
A method of forming a seal structure therefore needs to be developed wherein the seal structure has a lower peel initiation force when compared to a conventional adhesive seal system, as well as a lower average peel force. Such a seal structure should not, however, sacrifice any high burst pressure capabilities for the benefit of the lower peel initiation and lower average peel forces.